Cook the Potatoes
Place potatoes in a large pot and cover with cold water.
Add about 1 tablespoon of salt.
Bring to a boil, then reduce to a gentle boil for 15–20 minutes, or until fork tender.
Drain well and let them sit in the warm pot for 2–3 minutes to evaporate excess moisture.
Prepare the Greens
While the potatoes cook, melt 2 tablespoons butter in a large skillet.
Add the cabbage (or kale) and cook for 5–8 minutes until tender.
Stir in the scallions during the last 2 minutes.
If using garlic, add it during the final minute of cooking.
Warm the Milk
Heat the milk gently in a small saucepan or microwave until warm (not boiling).
Mash Everything Together
Mash the potatoes until smooth.
Add the remaining 2 tablespoons butter and gradually stir in the warm milk.
Fold in the cooked cabbage and scallions.
Season with salt and black pepper to taste.
Serve
Spoon into a serving bowl.
Make a small well in the center and add a pat of butter to melt over the top.
Garnish with parsley if desired.